It is known that steel structures built in marine climate regions are greatly damaged due to corrosion resulting from seawater and sea-salt particles. Consequently, efforts have been made to provide some sort of corrosion protection surface treatment on these steel structures, in order to inhibit corrosion. Painting, hot-dip galvanizing and hot-dip aluminizing are representative.
Other methods include forming a sprayed coating of zinc or aluminum, or alloys of these, which exhibits an electrochemically base potential, by way of thermal spraying. However, while conventional corrosion protection techniques based on sprayed coatings of metals such as zinc and aluminum on steel structures were found to have some corrosion protection effect, because the steel structures were protected by way of a sacrificial anodic effect, periodic respraying was necessary (see Patent Literature 1 and 2, below).
Thus, it has recently become more common to perform thermal spraying with corrosion-resistant alloys such as Al—Mg alloys, which can impart better characteristics for salt water environments than zinc and aluminum.
However, there is a problem insomuch as, because sprayed coatings become porous, they are readily subject to salt damage, and thus the rate of corrosion damage to the steel structures is high. Further, in ordinary thermally sprayed coatings, when the fine particles of the sprayed material fly out in the sprayed gas stream, they are flattened, so as to form a laminated structure on the substrate, together with which the individual particles are heated and fused, and at this time they are oxidized by the air in the spraying atmosphere, such that oxide films necessarily form on the surfaces thereof, which results in the formation of minute voids, mediated by the oxide films, and these voids become routes through which aqueous solutions such as seawater penetrate into the interior.
For this reason, in locations having aggressive saltwater environments such as coastal regions, the durability of coatings is conventionally no more than a few dozen years, even with corrosion protection methods based on Al—Mg spraying. Considering the infrastructure development environment that is associated with the future decreasing population of Japan, there is now a strong demand for these to last longer, for example on the scale of 100 years.